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GRACE

Mission Specification

The Gravity Recovery and Climate Experiment (GRACE) twin satellites, launched 17 March 2002, are making detailed measurements of Earth's gravity field and improving investigations about Earth's water reservoirs, over land, ice and oceans.

GRACE measures gravity by relating it to the distance between the 2 satellites. When there is an increase in gravity ahead of the pair, the front satellite speeds up and the distance between the pair increases, when the increased gravity is between the pair their distance decreases; the opposite occurs when there is decreased gravity ahead of, or between the satellite pair.
The satellites are separated by 220 km and they can detect changes smaller than a micrometer per second in relative velocity. These measurements, in conjunction with other data and models, have provided observations of terrestrial water storage changes, ice-mass variations, ocean bottom pressure changes and sea-level variations.

Since the launch of GRACE the orbit has been slowly decaying due to the atmospheric drag on the satellites. The orbit parameters given in the orbit section are from the beginning of the mission. For the current orbit parameters please visithttp://www.csr.utexas.edu/grace/.

Since December 2010 the batteries on GRACE are feeling their age and are not capable of retaining a full charge. To extend the battery life data are not collected when GRACE is eclipsed and cannot collect solar energy; when GRACE is not eclipsed it collects data normally.

GRACE ground segment operations are co-funded by ESA. Therefore, ESA is supporting the continuation of the measurements of mass redistribution in the Earth system.

Guide to Available Data

(December 2, 2014)
Distribution of Release 05 (RL05) began March 2012. Errors in the AOD1B de-aliasing fields from 26 June 2013 to 31 July 2014 were corrected, AOD1B data in the archive were replaced on 3 October 2014. The CSR Level 2 RL05 fields from June 2013 to June 2014 based on these erroneous de-aliasing fields were replaced on 6 October 2014. The GFZ Level 2 RL05 fields were replaced on 26 September 2014. If you have downloaded any of these products before these dates, please download the replacement files.

The newest Level-1B and Level-2 products are made available at approximately monthly intervals. Level-1B data refer to the collection of ranging, accelerometry, attitude and related ancillary data. All products have been created by the NASA/JPL element of the SDS.

Due to errors in the AOD1B de-aliasing fields from 26 June 2013 to 31 July 2014, these AOD1B fields have been corrected and the CSR level 2 data reprocessed. If you downloaded any Release 05 CSR data product for June 2013 through June 2014 before 6 October 2014, please download those files again.

Due to errors in the AOD1B de-aliasing fields from 26 June 2013 to 31 July 2014, these AOD1B fields have been corrected and the GFZ level 2 data reprocessed. If you downloaded any Release 05 GFZ data product for June 2013 through June 2014 before 26 September 2014, please download those files again.

Due to errors in the AOD1B de-aliasing fields from 26 June 2013 to 31 July 2014, these AOD1B fields have been corrected but the JPL level 2 data have not been reprocessed. If you downloaded any Release 05 JPL data product for June 2013 through June 2014, please be aware that those data have known problems and have been replaced by JPL Release 05.1, which uses the corrected de-aliasing fields.

If you require gridded/mapped GRACE data or do not want to generate the gravity fields from the spherical harmonics please visit the GRACE Tellus website at http://grace.jpl.nasa.gov/

Instruments

ACC – Super STAR Accelerometer is in both satellites and measures non-gravitational forces acting on the satellites, such as solar radiation pressure, air drag and attitude control activator operation.

October 6, 2014: Errors in the AOD1B de-aliasing fields from 26 June 2013 to 31 July 2014 were corrected and AOD1B data in the archive were replaced on 3 October 2014. The CSR Level 2 RL05 fields from June 2013 to June 2014 based on these erroneous de-aliasing fields were replaced on 6 October 2014. The GFZ Level 2 RL05 fields were replaced on 26 September 2014. If you have downloaded any of these products before these dates, please download the replacement files.

July 17, 2012 Replacement CSR-GSM: The CSR-GSM products have been replaced for the following four months: July-2004; October-2004; March-2005 and February-2006. The GSM products for these four months have been updated with the refined data editing. While the GAC/GAD products for these four months are unchanged, they have been updated as well, for consistency. If you downloaded the data products for these four months prior to July 17, 2012, please download the replacement products. No other products previously delivered are so affected.

Replacement SLR-derived C20 values: The RL05-compatiblevalues of SLR-derived C20 harmonics are provided in the new Technical Note 07 (TN07), available along with other Technical Notes through the GRACE data archives CSR & JPL GRACE-SDSrecommends that users should replace the C20 values within the CSR/JPL-RL05 GSM files with the corresponding values from TN07. GFZ GRACE-SDS recommends that users should use as-is the C20 values within the GFZ-RL05 GSM files, and should not replace them with SLR-derived estimates. The TN07 values of SLR-derived C20 are not inter-changeable with the TN05 values (for RL04), due to changes in background models, and due to an absence of background rates modeled in the former. http://www.csr.utexas.edu/grace/RL05.html

Feb 29, 2008 (NOTE-3): The influence of the error in AOD1B product, described in NOTE-1, on the Level-2 gravity field products, though small, is not negligible. Therefore all GSM-products between June 2006 and September 2007 have been reprocessed with the correct AOD1B. Click Here for a summary of the principal changes in the Level-2 fields in this duration.